Numerous studies of the nitrogen cycle in the upper ocean have shown that the growth of oceanic phytoplankton is often largely supported by the supply of ammonium excreted by herbivorous zooplankton and members of other tropic levels. Despite considerable knowledge of the growth and metabolic responses of phytoplankton and herbivorous zooplankton to supplies of energy and nutrients, no general quantitative description of nitrogen cycling in the planktonic community has been proposed. Here we present a hypothetical description of a closed, steady state, and well mixed water column in which nitrogen is cycled between a nutrient pool, the crop of phytoplankton, and the stock of zooplankton. The description is based upon the mathematical models of growth and metabolic regulation of phytoplankton (Kiefer and Mitchell, 1983) and of a small macroherbivore (Lehman, 1976). In order to resolve the system's state, we propose that the stable system achieves an optimal efficiency in the conversion of the radiant energy into the chemical energy stored in herbivore biomass. We have solved the equations for such a system in which the light fields and total concentrations of nitrogen within the mixed layer are varied. The response of the phytoplankton to variations in radiant energy is different from the responses of the herbivores; the phytoplankton vary their growth rates but maintain crop size while the herbivores vary stock size but maintain growth rate. On the other hand, the response of the phytoplankton and herbivores to variations in nitrogen concentration within the mixed layer are similar: the growth rates and standing stocks of both the phytoplankter and herbivore increase with nitrogen concentration until saturation. A preliminary comparison of these predictions with oceanographic observations indicates consistency.